Wire bonding machines are often used in forming a first wire bond on a contact pad of a semiconductor chip, extending the wire from the contact pad to a lead finger for a package lead (e.g., in a leadframe), forming a second wire bond on the lead finger, and then severing the wire near the second wire bond. This process may be repeated a rate of about 10 times per second during the fabrication of a packaged semiconductor chip, for example.
A typical wire bonding machine has a bond head that feeds the wire through a capillary and that moves the capillary relative to the chip and leadframe to form the wire bonds. During a typical wire bonding process, as described above, the wire is fed due to the motion of the bond head relative to a wire bond just formed. For example, after a first wire bond is performed on a chip contact pad, the end of the wire is attached to the chip contact pad. Movement of the bond head relative to the chip contact pad (where the wire tip is attached) causes the wire to be pulled out of the bond head through the capillary. Hence, at this point, the wire is not constrained from being fed out of the capillary by the bond head. After the wire is bonded to a lead finger (e.g., by pressing the wire against the lead finger with the capillary), the wire is typically severed. In a conventional wire bonding machine, the wire is severed at this time by clamping onto the wire within the bond head while moving the bond head relative to the second wire bond (where the wire is attached to the lead finger, for example). Such action causes the wire to sever at or near the second wire bond site. Typically, a short length of the wire is allowed to be drawing out of the capillary before clamping onto the wire to cause it to sever at the second wire bond. This short length of wire left extending from the tip of the capillary after severing the wire is often referred to as a pigtail. The pigtail is then exposed to a flame to cause the wire tip to melt. Such melting of the wire tip causes the wire tip to have a ball shape. Note that, although very common, the wire bonding process just described is just one example wire bonding process, and many other wire bonding processes may exist.
When the wire is clamped for initiating the severing of the wire, the clamping action in a conventional wire bonding machine is usually provided by two flat plates of a clamp that move toward each other and sandwich part of the wire there between. When it is desired to allow the wire to be drawn out of the capillary, the clamp is opened and left open (i.e., the flat plates move away from each other) to allow the wire to slide relatively freely between the flat plates. A downside to this conventional design, however, is that the flat plates often flatten a portion of the wire where the wire is clamped upon by the flat plates, which may be undesirable. Hence, there is a need for a way for a bond head to clamp or hold the wire without damaging or flattening the wire at the location where the wire is held.